The mitochondrial permeability transition (MPT) is a sudden increase of the inner membrane permeability to solutes with molecular mass below approximately 1500 Da with consequent mitochondrial swelling and release of cytochrome C. The key mitochondrial mechanism causing MPT is opening of the permeability transition pore (PTP), a high-conductance inner membrane channel whose entire molecular components are still to be fully identified (Forte, M.; Bernardi, P. Genetic dissection of the permeability transition pore. J. Bioenerg. Biomembr., 2005, 37, 121-8).
The PTP open state dissipates the proton electrochemical gradient resulting in Ca2+ release and ATP hydrolysis and is a potential cause of osmotic swelling. This in turn can cause inner membrane unfolding and outer membrane rupture followed by release of proapoptotic intermembrane proteins into the cytosol. The PTP open-closed state is regulated by multiple effectors acting at various sites and many of these sites are affected by conditions and mediators implied in a variety of models of cell death (Bernardi, P. Mitochondrial transport of cations: Channels, exchangers and permeability transition. Physiol. Rev., 1999, 79, 1127-1155). Hence PTP opening is one mitochondria-mediated mechanism for promoting cell death.
Programmed cell death (apoptosis) is an essential process of multicellular organisms and the physiological rate of apoptosis controls normal development and tissue renewal. However, an increased rate of apoptosis is found in many pathological conditions characterized by excessive cell loss and tissue degeneration. A wide number of cellular stresses cause functional and structural changes to mitochondria that, through cell death, impinge on tissue homeostasis and functionality (Green, D. R.; Kroemer, G. The pathophysiology of mitochondrial cell death, Science, 2004, 305, 626-629). An increased rate of apoptosis in stem cell compartments affects tissue renewal and accelerates aging whilst a reduction of functioning cell mass impairs tissue performance directly (Pelicci, P. G. Do tumor-suppressive mechanisms contribute to organism aging by inducing stem cell senescence? J. Clin. Invest., 2004, 113, 4-7).
Mitochondria play a central role in triggering apoptosis. The most critical mitochondrial events during apoptosis are the structural and functional remodelling of this organelle and subsequent release of apoptogenic proteins from the mitochondrial intermembrane space into the cytosol. These proteins include cytochrome C, Smac/DIABLO, AIF, Omi/HtrA2. Further, strong evidence exists to suggest that PTP opening may be an early event in the commitment to apoptosis.
The role of mitochondrial apoptosis in the aetiology of many diseases is well established and the increased rate of apoptosis typical of pathological stress conditions which are observed during myocardial infarction, renal ischemia, type I and Type II diabetes or neurodegenerative diseases always correlates with the Mitochondrial Permeability Transition and loss of mitochondrial integrity. Many of these disease states are characterized by significant increase of reactive oxygen species (ROS) that are known inducers of the MPT and apoptosis.
Some of the most important diseases resulting from opening of the MPTP are listed below:
Heart Infarction
In ischemic heart disease, sequential ischemia-reperfusion events occur resulting in myocardium cell death by necrosis and/or apoptosis and a role for the PTP and mitochondrial swelling in myocardial infarction has already been demonstrated (Solaini, G.; Harris, D. A. Biochemical dysfunction in heart mitochondria exposed to ischemia and reperfusion. Biochem. J., 2005, 390, 377-394). Disruption of mitochondrial integrity in cardiomyocytes through the deregulation of Ca2+ and the production of ROS as a consequence of hypoxia-hyperoxia transition, is the major trigger of apoptosis during myocardial infarction. (Di Lisa, F.; Bernardi, P. Mitochondria and ischemia-reperfusion injury of the heart: Fixing a hole. Cardiovasc. Res., 2006, 70, 2, 191-199). Furthermore, cyclosporine A (CsA), a known inhibitor of the PTP, has been shown to improve recovery of baseline myocardial contractile function in human atrial tissue after experimental hypoxia and reperfusion.
Neurological Diseases and Stroke
PTP and mitochondrial swelling are also involved in neuronal cell death following degenerative diseases and in many high-prevalence conditions of brain damage such as hyperglycaemia, hypoglycaemia, stroke, ischemia, trauma and in experimental epilepsy (Pordan, J.; Cena, V.; Prehn, J. H. Mitochondrial control of neuron death and its role in neurodegenerative disorders. J. Physiol. Biochem., 2003, 59, 129-141; Li, P. A.; Uchino, H.; Elmer, E.; Siesjö, B. K. Amelioration by cyclosporin A of brain damage following 5 or 10 min of ischemia in rats subjected to preischemic hyperglycaemia. Brain Res., 1997, 753, 133-140; Folbergrova, J.; Li, P. A.; Uchino, H.; Smith, M. L.; Siesjö, B. K. Changes in the bioenergetic state of rat hippocampus during 2.5 min of ischemia, and prevention of cell damage by cyclosporin A in hyperglycaemic subjects. Exp. Brain Res., 1997, 114, 44-50; Friberg, H.; Ferrand-Drake, M.; Bengtsson, F.; Halestrap, A. P.; Wieloch, T. Cyclosporin A, but not FK 506, protects mitochondria and neurons against hypoglycaemic damage and implicates the mitochondrial permeability transition in cell death. J. Neurosci., 1998, 18, 5151-5159; Sims, N. R. and Anderson M. F. Mitochondrial contributions to tissue damage in stroke. Neurochem Int., 2002, 40, 511-26; Stavrovskaya I. G.; Narayanan M. V.; Zhang W.; Krasnikov B. F.; Heemskerk J.; Young S. S.; Blass J. P.; Brown A. M.; Beal M. F.; Friedlander R. M.; Kristal B. S. Clinically approved heterocyclics act on a mitochondrial target and reduce stroke-induced pathology. J Exp Med., 2004, 200, 211-22; Matsumoto, S.; Friberg, H.; Ferrand-Drake, M.; Wieloch, T. Blockade of the mitochondrial permeability transition pore diminishes infarct size in the rat after transient middle cerebral artery occlusion. J. Cereb. Blood Flow Metab., 1999, 19, 736-741 Scheff, S. W.; Sullivan P. G. Cyclosporin A significantly ameliorates cortical damage following experimental traumatic brain injury in rodents. J. Neurotrauma, 1999, 16, 783-792; Kudin, A. P.; Debska-Vielhaber, G.; Vielhaber, S.; Elger, C. E.; Kunz, W. S. The mechanism of neuroprotection by topiramate in an animal model of epilepsy. Epilepsia, 2004, 45, 1478-1487). The beta-amyloid protein responsible for Alzheimer's disease induces oxidative stress and mitochondrial damage resulting in neuronal cell death indicating that mitochondrial dysfunction is associated with disease progression (Abramov, A. Y.; Canevari, L.; Duchen, M. R. Beta-amyloid peptides induce mitochondrial dysfunction and oxidative stress in astrocytes and death of neurons through activation of NADPH oxidase. J. Neurosci., 2004, 24, 565-575). In addition, the massive apoptosis occurring in the dopaminergic cells in Parkinson's disease has been causally linked to the MTP (Fiskum, G.; Starkov A.; Polster, B. M.; Chinopoulos, C. Mitochondrial mechanisms of neural cell death and neuroprotective interventions in Parkinson's disease. Ann. N.Y. Acad. Sci. 2003, 991, 111-119). Further, mitochondrial dysfunction leading to MPT has been characterized in Huntington's disease and amyotrophic lateral sclerosis (Tang, T. S.; Slow, E.; Lupu, V.; Stavrovskaya, I. G.; Sugimori, M.; Llinas, R.; Kristal, B. S.; Hayden, M. R.; Bezprozvanny, I. Disturbed Ca2+ signaling and apoptosis of medium spiny neurons in Huntington's disease. Proc. Natl. Acad. Sci. U.S.A., 2005, 102, 2602-2607; Kirkinezos, I. G.; Hernandez, D.; Bradley, W. G.; Moraes, C. T. An ALS mouse model with a permeable blood-brain barrier benefits from systemic cyclosporine A treatment. J. Neurochem., 2004, 88, 821-826).
Diabetes
Diabetes, including type I and type II diabetes, induces damage and cell death by several mechanisms. Above all, hyperglycaemia itself is able to directly kill cells in a variety of tissues and in kidney, endothelial and retinal cell cultures (Allen, D. A.; Yaqoob, M. M.; Harwood, S. M. Mechanisms of high glucose-induced apoptosis and its relationship to diabetic complications. J. Nutr. Biochem., 2005, 16, 705-713). Diabetic retinopathy (DR) is one of the peripheral micro vascular complications strongly enhancing the morbidity of diabetic vascular diseases. This progressive condition leading to retinal neovascularization, the most common cause of blindness among young patients, has an enormous clinical significance in developed countries.
DR begins with an early pre-proliferative stage (background retinopathy) characterized by loss of capillary pericytes, progressive capillary closure, micro-aneurisms and retinal oedema. The subsequent retinal ischemia (or hypoxia) due to vessel occlusion, triggers abnormal retinal vessel growth. Neo-vessels extend along the inner surface of the retina and/or into the vitreous cavity and can lead to retinal detachment and haemorrhage. This stage is known as proliferative diabetic retinopathy (PDR). Hyperglycemic stress is considered a key factor in PDR since it induces increased production of vascular endothelium growth factor (VEGF) by retinal cells leading to neovascularization and causes cellular oxidative damage having repercussions on the mitochondria. In this view, mechanisms such as oxidative stress seem to play a major role in the pathogenic sequence. ROS, formed in higher amounts during diabetes, could trigger most of the pathologic intracellular pathways involved in PDR and it has been demonstrated that ROS are produced in retina during reperfusion following diabetes-induced ischemia. In addition, ROS have been implicated in the loss of micro-vascular pericytes, one of early alterations in the background diabetic retinopathy. A recent report demonstrated that retinal pericytes died through apoptosis if subjected to either hydrogen peroxide or ultraviolet radiation (UV), both of which are sources of ROS. Finally, oxidative stress was also correlated with incidence and progression of retinopathy of prematurity (ROP). The immature retina contains relatively low levels of antioxidants such as heme-oxygenase-1 and catalase. During hyper-oxygenation ROS are produced and, among other things, favour the generation of biologically active isoprostanes concurring to ischemia and, therefore, to the pathogenesis of ROP.
Inherited Dystrophies
Genetic disorders associated with tissue dysfunction and instability are often characterized by an increased rate of apoptosis. Indeed MPT was found to be causally involved in myocyte loss in some inherited myopathies. In particular inhibiting PTP with CsA treatment has been shown to reduce myofiber degeneration and to cure collagen type VI myopathy in mice (Irwin, W. A.; Bergamin, N.; Sabatelli, P.; Reggiani, C.; Megighian, A.; Merlini, L.; Braghetta, P.; Columbaro, M.; Volpin, D.; Bressan, G. M.; Bernardi, P.; Bonaldo, P. Mitochondrial dysfunction and apoptosis in myopathic mice with collagen VI deficiency. Nat. Genet., 2003, 35, 367-371).
Hepatitis
Liver can be damaged by different agents such as chemical poisons, inflammatory factors or viruses. In all cases, hepatocytes undergo massive apoptosis that is driven by the MPT (Haouzi, D.; Cohen, I.; Vieira, H. L.; Poncet, D.; Boya, P.; Castedo, M.; Vadrot, N.; Belzacq, A. S.; Fau, D.; Brenner, C.; Feldmann, G.; Kroemer, G. Mitochondrial permeability transition as a novel principle of hepatorenal toxicity in vivo. Apoptosis., 2002, 7, 395-405; Shirakata, Y.; Koike, K. Hepatitis B virus X protein induces cell death by causing loss of mitochondrial membrane potential. J. Biol. Chem., 2003, 278, 22071-22078). Furthermore, it has been reported that inhibiting PTP opening by treating mice with CsA strongly reduces liver damage in a rat model of TNF-α-dependent acute inflammatory hepatitis (Soriano, M. E.; Nicolosi, L.; Bernardi P. Desensitization of the permeability transition pore by cyclosporine A prevents activation of the mitochondrial apoptotic pathway and liver damage by tumour necrosis factor-alpha. J. Biol. Chem., 2004, 279, 36803-36808). Further, thiazole derivatives, as inhibitors of the PTP, were reported as being useful as medicines for preventing or treating hepatic cyrrhosis (Auget, M. et al., WO03009843).
In summary the mitochondrial permeability transition has been shown to be involved in the mechanism of apoptosis in a variety of disease states characterized by degenerative tissue damage. Hence pharmacological inhibition of the MPT represents an important approach for the prevention and treatment of a wide spectrum of diseases including cardiac infarction, neurological disease (due, for example, to traumatic insults and autoimmune reaction) and stroke, diabetes and diabetic complications such as diabetic retinopathy, inherited dystrophies and hepatitis (Kroemer, G. The mitochondrial permeability transition pore complex as a pharmacological target. Curr. Med. Chem., 2003, 10, 1469-1472; Mattson, M. P.; Kroemer, G. Mitochondria in cell death: novel targets for neuroprotection and cardioprotection. Trends Mol. Med., 2003, 9, 196-205).